Many electronic systems include circuits, such as switching power converters or transformers that interface with a dimmer The interfacing circuits deliver power to a load in accordance with the dimming level set by the dimmer For example, in a lighting system, dimmers provide an input signal to a lighting system. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers generate an output signal in which a portion of an alternating current (“AC”) input signal is removed or zeroed out. For example, some analog-based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current supply voltage. This modulation of the phase angle of the supply voltage is also commonly referred to as “phase cutting” the supply voltage. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load.
A particular type of a triac-based, phase-cutting dimmer is known as a leading-edge dimmer A leading-edge dimmer phase cuts from the beginning of an AC cycle, such that during the phase-cut angle, the dimmer is “off” and supplies no output voltage to its load, and then turns “on” after the phase-cut angle and passes phase cut input signal to its load. To ensure proper operation, the load must provide to the leading-edge dimmer a load current sufficient to maintain an inrush current above a current necessary for opening the triac. Due to the sudden increase in voltage provided by the dimmer and the presence of capacitors in the dimmer, the current that must be provided is typically substantially higher than the steady state current necessary for triac conduction. Additionally, in steady state operation, the load must provide to the dimmer a load current to remain above another threshold known as a “hold current” needed to prevent premature disconnection of the triac.
FIG. 1 depicts a lighting system 100 that includes a triac-based leading-edge dimmer 102 and a lamp 142. FIG. 2 depicts example voltage and current graphs associated with lighting system 100. Referring to FIGS. 1 and 2, lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Triac 106 acts as a voltage-driven switch, and a gate terminal 108 of triac 106 controls current flow between the first terminal 110 and the second terminal 112. A gate voltage VG on the gate terminal 108 above a firing threshold voltage value VF will cause triac 106 to turn ON, in turn causing a short of capacitor 121 and allowing current to flow through triac 106 and dimmer 102 to generate an output current iDIM.
Assuming a resistive load for lamp 142, the dimmer output voltage VΦ_DIM is zero volts from the beginning of each of half cycles 202 and 204 at respective times t0 and t2 until the gate voltage VG reaches the firing threshold voltage value VF. Dimmer output voltage VΦ_DIM represents the output voltage of dimmer 102. During timer period tOFF, the dimmer 102 chops or cuts the supply voltage VSUPPLY so that the dimmer output voltage VΦ_DIM remains at zero volts during time period tOFF. At time t1, the gate voltage VG reaches the firing threshold value VF, and triac 106 begins conducting. Once triac 106 turns ON, the dimmer voltage VΦ_DIM tracks the supply voltage VSUPPLY during time period tON.
Once triac 106 turns ON, the current iDIM drawn from triac 106 must exceed an attach current iATT in order to sustain the inrush current through triac 106 above a threshold current necessary for opening triac 106. In addition, once triac 106 turns ON, triac 106 continues to conduct current iDIM regardless of the value of the gate voltage VG as long as the current iDIM remains above a holding current value iHC. The attach current value iATT and the holding current value iHC is a function of the physical characteristics of the triac 106. Once the current iDIM drops below the holding current value iHC, i.e. iDIM<iHC, triac 106 turns OFF (i.e., stops conducting), until the gate voltage VG again reaches the firing threshold value VF. In many traditional applications, the holding current value iHC is generally low enough so that, ideally, the current iDIM drops below the holding current value iHC when the supply voltage VSUPPLY is approximately zero volts near the end of the half cycle 202 at time t2.
The variable resistor 114 in series with the parallel connected resistor 116 and capacitor 118 form a timing circuit 115 to control the time t1 at which the gate voltage VG reaches the firing threshold value VF. Increasing the resistance of variable resistor 114 increases the time tOFF, and decreasing the resistance of variable resistor 114 decreases the time tOFF. The resistance value of the variable resistor 114 effectively sets a dimming value for lamp 142. Diac 119 provides current flow into the gate terminal 108 of triac 106. The dimmer 102 also includes an inductor choke 120 to smooth the dimmer output voltage VΦ_DIM. Triac-based dimmer 102 also includes a capacitor 121 connected across triac 106 and inductor choke 120 to reduce electro-magnetic interference.
Ideally, modulating the phase angle of the dimmer output voltage VΦ_DIM effectively turns the lamp 142 OFF during time period tOFF and ON during time period tON for each half cycle of the supply voltage VSUPPLY. Thus, ideally, the dimmer 102 effectively controls the average energy supplied to lamp 142 in accordance with the dimmer output voltage VΦ_DIM.
The triac-based dimmer 102 adequately functions in many circumstances, such as when lamp 142 consumes a relatively high amount of power, such as an incandescent light bulb. However, in circumstances in which dimmer 102 is loaded with a lower-power load (e.g., a light-emitting diode or LED lamp), such load may draw a small amount of current iDIM, and it is possible that the current iDIM may fail to reach the attach current iATT and also possible that current iDIM may prematurely drop below the holding current value iHC before the supply voltage VSUPPLY reaches approximately zero volts. If the current iDIM fails to reach the attach current iATT, dimmer 102 may prematurely disconnect and may not pass the appropriate portion of input voltage VSUPPLY to its output. If the current iDIM prematurely drops below the holding current value iHC, the dimmer 102 prematurely shuts down, and the dimmer voltage VΦ_DIM will prematurely drop to zero. When the dimmer voltage VΦ_DIM prematurely drops to zero, the dimmer voltage VΦ_DIM does not reflect the intended dimming value as set by the resistance value of variable resistor 114. For example, when the current iDIM drops below the holding current value iHC at a time significantly earlier than t2 for the dimmer voltage VΦ_DIM 206, the ON time period tON prematurely ends at a time earlier than t2 instead of ending at time t2, thereby decreasing the amount of energy delivered to the load. Thus, the energy delivered to the load will not match the dimming level corresponding to the dimmer voltage VΦ_DIM. In addition, when VΦ_DIM prematurely drops to zero, charge may accumulate on capacitor 118 and gate 108, causing triac 106 to again refire if VG exceeds VF during the same half cycle 202 or 204, and/or causing triac 106 to fire incorrectly in subsequent half cycles due to such accumulated charge. Thus, premature disconnection of triac 106 may lead to errors in the timing circuitry of dimmer 102 and instability in its operation.
Dimming a light source with dimmers saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. However, conventional dimmers, such as a triac-based leading-edge dimmer, that are designed for use with resistive loads, such as incandescent light bulbs, often do not perform well when attempting to supply a raw, phase modulated signal to a reactive load such as an electronic power converter or transformer.
Transformers present in a power infrastructure may include magnetic or electronic transformers. A magnetic transformer typically comprises two coils of conductive material (e.g., copper) each wrapped around of core of material having a high magnetic permeability (e.g., iron) such that magnetic flux passes through both coils. In operation an electric current in the first coil may produce a changing magnetic field in the core, such that the changing magnetic field induces a voltage across the ends of the secondary winding via electromagnetic induction. Thus, a magnetic transformer may step voltage levels up or down while providing electrical isolation in a circuit between components coupled to the primary winding and components coupled to the secondary winding.
On the other hand, an electronic transformer is a device which behaves in the same manner as a conventional magnetic transformer in that it steps voltage levels up or down while providing isolation and can accommodate load current of any power factor. An electronic transformer generally includes power switches which convert a low-frequency (e.g., direct current to 400 Hertz) voltage wave to a high-frequency voltage wave (e.g., in the order of 10,000 Hertz). A comparatively small magnetic transformer may be coupled to such power switches and thus provides the voltage level transformation and isolation functions of the conventional magnetic transformer.
FIG. 3 depicts a lighting system 101 that includes a triac-based leading-edge dimmer 102 (e.g., such as that shown in FIG. 1), a magnetic transformer 122, and a lamp 142. Such a system may be used, for example, to transform a high voltage (e.g., 110V, 220 V) to a low voltage (e.g., 12 V) for use with a halogen lamp (e.g., an MR16 halogen lamp). FIG. 4 depicts example voltage and current graphs associated with lighting system 101. Referring to FIGS. 3 and 4, when dimmer 102 is used in connection with transformer 122 and a low-power lamp 142, the low power draw of lamp 142 may cause insufficient current iDIM to be drawn from dimmer 102 in order to meet the attach current and/or hold current requirements.
To further illustrate this potential problem, an equivalent circuit model for transformer 122 that represents the physical behavior of a magnetic transformer is depicted in FIG. 3. Parasitic effects present in transformer 122 are represented in an equivalent circuit model for transformer 122 by a primary side parasitic inductance 124 (with an inductance Lp) in series with a primary side parasitic resistance 126 (with a resistance Rp) and a secondary side parasitic inductance 132 (with an inductance Ls) in series with a secondary side parasitic resistance 134 (with an resistance Rs), which model losses and leakage reactances of the transformer coils. Parasitic effects are also represented by a “magnetizing branch” of the model comprising shunt leg parasitic inductance 128 (with an inductance Lm) in parallel with a shunt leg parasitic resistance 130 (with a resistance Rm), which model losses and leakage reactances of the transformer core. A magnetizing current Im flows to the shunt leg reactance representing current required to maintain mutual magnetic flux in the core. Those of ordinary skill in the art will appreciate that iDIM=is/N+im, where is is a secondary current of transformer 122 and N is the turns ratio of the transformer's primary and second side windings.
FIG. 4 depicts example waveforms for dimmer 102 output voltage VΦ_DIM 402, secondary voltage Vs 404, magnetizing current im 406, and the current is/N 408 through the primary winding of transformer 122, assuming a three-wire dimmer. When loaded with transformer 122, the waveform VΦ_DIM 402 shown in FIG. 4 differs from that of waveform VΦ_DIM 206 shown in FIG. 2 due to reactances present in transformer 122, and in particular the presence of magnetizing current Im. Starting at time t0 in half cycle 410, despite a zero voltage VΦ_DIM at t0, a magnetizing current im 406 remains flowing in transformer 122 and may account for significantly all of the current iDIM, thus inducing a voltage Vs 404 rising in magnitude. At such time t0, primary winding current is/N may also begin increasing above zero. At a time t3 occurring between time t0 and time t1, the sum of magnetizing current im and primary winding current is/N may reach a point at which the sum iDIM=is/N+im will decrease to an amount below the hold current iHC, and dimmer 102 turns off. At time t1, the dimmer may again turn on (e.g., iDIM>iATT), and a magnetizing current im and primary winding current is/N may again appear. As seen in FIG. 4, throughout half cycle 412, waveforms 402, 404, and 408 are substantially equal in magnitude than they are throughout half cycle 410, but with opposite polarity. Accordingly, at a time t3′ occurring between time t2 and t1′, the sum of magnetizing current im and primary winding current is/N may reach a point at which the sum iDIM=is/N+im will decrease to an amount below the hold current iHC, and dimmer 102 turns off. In a three-wire dimmer, the time at which the dimmer turns on (e.g., t1, t1′) within each phase remains consistent, while in a two-wire dimmer, such times may vary from phase to phase.
Another particular type of phase-cutting dimmer is known as a trailing-edge dimmer A trailing-edge dimmer phase cuts from the end of an AC cycle, such that during the phase-cut angle, the dimmer is “off” and supplies no output voltage to its load, but is “on” before the phase-cut angle and in an ideal case passes a waveform proportional to its input voltage to its load.
FIG. 5 depicts a lighting system 500 that includes a trailing-edge, phase-cut dimmer 502 and a lamp 542. FIG. 6 depicts example voltage and current graphs associated with lighting system 500. Referring to FIGS. 5 and 6, lighting system 500 receives an AC supply voltage VSUPPLY from voltage supply 504. The supply voltage VSUPPLY, indicated by voltage waveform 602, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Trailing edge dimmer 502 phase cuts trailing edges, such as trailing edges 602 and 604, of each half cycle of supply voltage VSUPPLY. Since each half cycle of supply voltage VSUPPLY is 180 degrees of the supply voltage VSUPPLY, the trailing edge dimmer 502 phase cuts the supply voltage VSUPPLY at an angle greater than 0 degrees and less than 180 degrees. The phase cut, input voltage VΦ_DIM to lamp 542 represents a dimming level that causes the lighting system 500 to adjust power delivered to lamp 542, and, thus, depending on the dimming level, increase or decrease the brightness of lamp 542.
Dimmer 502 includes a timer controller 510 that generates dimmer control signal DCS to control a duty cycle of switch 512. The duty cycle of switch 512 is a pulse width (e.g., times t1−t0) divided by a period of the dimmer control signal (e.g., times t3−t0) for each cycle of the dimmer control signal DCS. Timer controller 510 converts a desired dimming level into the duty cycle for switch 512. The duty cycle of the dimmer control signal DCS is decreased for lower dimming levels (i.e., higher brightness for lamp 542) and increased for higher dimming levels. During a pulse (e.g., pulse 606 and pulse 608) of the dimmer control signal DCS, switch 512 conducts (i.e., is “on”), and dimmer 502 enters a low resistance state. In the low resistance state of dimmer 502, the resistance of switch 512 is, for example, less than or equal to 10 ohms. During the low resistance state of switch 512, the phase cut, input voltage VΦ_DIM tracks the input supply voltage VSUPPLY and dimmer 502 transfers a dimmer current iDIM to lamp 542.
When timer controller 510 causes the pulse of dimmer control signal 606 to end, dimmer control signal 606 turns switch 512 off, which causes dimmer 502 to enter a high resistance state (i.e., turns off). In the high resistance state of dimmer 502, the resistance of switch 512 is, for example, greater than 1 kiloohm. Dimmer 502 includes a capacitor 514, which charges to the supply voltage VSUPPLY during each pulse of the timer control signal DCS. In both the high and low resistance states of dimmer 502, the capacitor 514 remains connected across switch 512. When switch 512 is off and dimmer 502 enters the high resistance state, the voltage VC across capacitor 514 increased (e.g., between times t1 and t2 and between times t4 and t5). The rate of increase is a function of the amount of capacitance C of capacitor 514 and the input impedance of lamp 542. If effective input resistance of lamp 542 is low enough, it permits a high enough value of the dimmer current iDIM to allow the phase cut, input voltage VΦ_DIM to decay to a zero crossing (e.g., at times t2 and t5) before the next pulse of the dimmer control signal DCS.
Dimming a light source with dimmers saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. However, conventional dimmers, such as a trailing-edge dimmer, that are designed for use with resistive loads, such as incandescent light bulbs, often do not perform well when supplying a raw, phase modulated signal to a reactive load such as a power converter or transformer, as is discussed in greater detail below.
FIG. 7 depicts a lighting system 500 that includes a trailing-edge, phase-cut dimmer 502, an electronic transformer 522, and a lamp 542. Such a system may be used, for example, to transform a high voltage (e.g., 110V, 220 V) to a low voltage (e.g., 12 V) for use with a halogen lamp (e.g., an MR16 halogen lamp). FIG. 8 depicts example voltage graphs associated with lighting system 501.
As is known in the art, electronic transformers operate on a principle of self-resonant circuitry. Referring to FIGS. 7 and 8, when dimmer 502 is used in connection with transformer 522 and a low-power lamp 542, the low current draw of lamp 542 may be insufficient to allow electronic transformer 522 to reliably self-oscillate.
To further illustrate, electronic transformer 522 may receive the dimmer output voltage VΦ_DIM at its input where it is rectified by a full-bridge rectifier formed by diodes 524. As voltage VΦ_DIM increases in magnitude, voltage on capacitor 526 may increase to a point where diac 528 will turn on, thus also turning on transistor 529. Once transistor 529 is on, capacitor 526 may be discharged and oscillation will start due to the self-resonance of switching transformer 530, which includes a primary winding (T2a) and two secondary windings (T2b and T2c). Accordingly, as depicted in FIG. 8, an oscillating output voltage Vs 800 will be formed on the secondary of transformer 532 and delivered to lamp 542 while dimmer 502 is on, bounded by an AC voltage level proportional to VΦ_DIM.
However, as mentioned above, many electronic transformers will not function properly with low-current loads. With a light load, there may be insufficient current through the switching transformer 530's primary to sustain oscillation. For legacy applications, such as where lamp 542 is a 35-watt halogen bulb, lamp 542 may draw sufficient current to allow transformer 522 to sustain oscillation. However, should a lower-power lamp be used, such as a six-watt LED bulb, the current drawn by lamp 542 may be insufficient to sustain oscillation in transformer 522, which may lead to unreliable effects, such as visible flicker and a reduction in total light output below the level indicated by the dimmer
In addition, traditional approaches do not effectively detect or sense a type of transformer to which a lamp is coupled, further rendering it difficult to ensure compatibility between low-power (e.g., less than twelve watts) lamps and the power infrastructure to which they are applied.